Inflatable chamber device for motion through a passage

ABSTRACT

A self-propelled device for locomotion through a lumen, comprising a set of serially arranged inflatable chambers, adjacent chambers being fluidly connected, and a fluid source attached to one end of the set, such that the chambers inflate sequentially. The chambers are constructed of an elastic material and have a wall thickness and dimensions such that they have a characteristic with a non-monotonous relationship between the inflation pressure within the chamber and the chamber&#39;s inflated size. The characteristic is such that after an initial inflation pressure peak, the non-monotonous relationship adopts a negative slope, such that the volume of the chamber increases more rapidly than the volume of fluid flowing into it, and the inflation pressure of the chamber falls. This effect causes the chamber to inflate and anchor rapidly, while essentially slowing down the inflation of the succeeding chamber until inflation of the first is complete.

FIELD OF THE INVENTION

The present invention relates to the field of inflatable chamber devicescapable of self propelled motion through tubes, especially forendoscopic and vascular use.

BACKGROUND OF THE INVENTION

The ability to crawl through long, flexible, and curved tubes has longbeen a challenge for engineers since numerous applications can benefitfrom a reliable solution. This ranges from medical applications fortreatment and diagnosis to sewer pipes, gas pipes and power plants.

Current solutions often contain a payload such as a camera, which ispushed from the back by a long flexible rod or wire. This is thesolution currently used in many medical applications with guide wires orcatheters to deliver diagnosis or treatment instruments to the desiredposition, e.g. in catheterization, colonoscopy, ureteroscopy, dilatingballoon, and others.

In some type of applications, it is impossible to push the active headfrom the back because the force required would cause buckling of thelong rod or wire. Additionally, one of the shortcomings of currentendoscopes and catheters is that as they are pushed into the passagewaymanually over a curved path, causing friction, there is a possibility ofinjury to the inner tissue walls of the passageway.

In search for a solution, a number of locomotion types of propulsionhave been developed, which pull at the distal end of the lumen ratherthen pushing at the proximal end. Examples in non-medical applicationsinclude crawling vehicles and spider-like robots. In medicalapplications, a common solution is that of the inch worm type, thatadvances by means of peristaltic motion, such as described in thearticle by P. Dario, et al., “Development and in vitro testing of aminiature robotic system for computer-assisted colonoscopy,” publishedin Computer Aided Surgery, Vol. 4, pp. 1-14, 1999, and in the article byJ. Dietrich et al., entitled “Development of a peristaltically actuateddevice for the minimal invasive surgery with a haptic sensor array”published in Micro- and Nano-structures of Biological Systems, Halle,Shaker-Verlag, 69-88. ISBN 3-8322-2655-9. Such devices are alsodescribed, for instance, in U.S. Pat. Nos. 6,764,441, 4,176,662,5,090,259, 5,662,587, 6,007,482, 5,364,353, and 6,702,735 and also inPCT Application No. PCT/IL2006/000925, to the inventors in the presentapplication.

Most of the above described devices have the disadvantage that a numberof control lines or pneumatic tubes are required to operate the device,which complicates both the control system and the physical deployment ofthe device within the passageway. The device described in theabove-mentioned U.S. Pat. No. 5,364,353 for “Apparatus for advancing anobject through a body passage” to M. T. Corfitsen et al., and in PCTApplication No. PCT/IL2006/000925, for “Tip propelled device for motionthrough a passage” to M. Shoham et al., on the other hand, require onlyone inflation tube. In U.S. Pat. No. 5,364,353, there is described adevice using a single bladder and an axially expandable bellows with athrottle valve between them. A tube is provided with a lumen for thesupply and removal of inflation medium to the bladder and bellows. Thethrottling valve ensures that the inflation of the bladder is delayedrelative to the axial expansion of the bellows as pressure is applied tothe inflation tube, and that the deflation of the bladder is delayedrelative to an axial contraction of the bellows as pressure is releasedfrom the inflation tube, such that the device can be advanced stepwisethrough, for instance, a gastrointestinal canal.

In PCT Application No. PCT/IL2006/000925, there is described a devicehaving a plurality of inflatable chambers arranged serially, andserially interconnected by means of small orifices, openings or tubesbetween adjacent chambers, in which at least the first and last chambersare expandable at least radially, and also optionally axially, and otherintermediate chambers, if present, are expandable at least axially andalso optionally radially. A tube is provided with a lumen for the supplyand removal of inflation medium to the chambers. The small orifices,openings or tubes ensure that the inflation of one chamber relative tothat preceding it is delayed, such that the chambers inflatesequentially as fluid is pumped into an inflation tube. Likewise, thedeflation of a chamber is delayed relative to that in front of it aspressure is released from the inflation tube, such that the device canbe advanced stepwise through, for instance, a gastrointestinal canal.

However, in practice, it is found that the control of the inflation anddeflation process is critically dependent on the fluid impedance of thesmall orifices, openings or tubes between the chambers, such that itbecomes difficult to obtain such a device which inflates and deflates,and thus, moves, at the desired rate. There thus exists a need for aninflatable balloon device, with a single inflation tube, in which thereis good control of the inflation sequence, such that an acceptably highrate of motion over internal passages can be obtained, and withoutcausing undue damage to the inner walls of the passages.

It is to be understood that the terms chamber, balloon, bladder andsimilar expressions used to describe the inflatable components of thevarious devices of the present application, may have been usedinterchangeably and even claimed thuswise, and it is to be understoodthat no difference is intended to be conveyed by use of one term or theother.

The disclosures of each of the publications mentioned in this sectionand in other sections of this application, are hereby incorporated byreference, each in its entirety.

SUMMARY OF THE INVENTION

The present invention seeks to provide a new method and device forefficient self-propulsion along internal passageways, of a locomotiondevice having a series of inflatable chambers in fluid connection bymeans of passageways between adjacent chambers, and with a singleinflation and deflation cycle to propel the device. The device utilizesthe dynamic behavior of fluid connected inflated chambers, whereby atime delay in the passage of inflating fluid from rearmost to foremostchamber is utilized to inflate the chambers in sequence beginning withthe rearmost, and ending with the foremost. Conversely, this same timedelay ensures that deflation of the chambers also proceeds in sequencebeginning with the rearmost, and ending with the foremost. Theinflatable chambers sequentially grip the inside wall of the passageway,starting with the chamber or chambers disposed at the rear or proximalend of the series while the device expands forward with inflation of theother chambers, and then gripping the inside wall of the passageway withthe chamber or chambers situated at the front or distal end of theseries, while the device pulls up its rear end with deflation of theother chambers. Instead of being supplied by means of a supply tube, thefluid source can be provided on board, such that the device is able tooperate independently of its surroundings.

The device is operable with a series of only two inflatable chambers,each of which expands radially and axially when inflated, but the use ofmore than two chambers may have an advantage in that the radial pressureon the walls is spread out over more chambers, thus reducing theinternal pressure required to anchor the relevant chambers of thedevice. Furthermore, the use of a larger number of chambers may enablelarger payloads to be transported or pulled by the device.

The device differs from prior art devices in that the elastic materialof the chambers or balloons and their dimensions are chosen to have acharacteristic providing a non-monotonous relationship between theinflation pressure within a chamber and its inflated size. When thematerial type, skin thickness and balloon dimensions are correctlyselected, such a characteristic has an initial pressure peak at acertain initial inflated size, beyond which, as the size of the chamberincreases, the internal pressure drops. This means that the elasticproperties of the chamber material are such as to allow the volume ofthe chamber to increases more quickly than the volume inflow ofinflating fluid, such that the chamber then continues to growconsiderably more rapidly than its rate of growth up to the initialpressure peak. The inflation scenario of the chambers thus acquires atwo-part characteristic. During the first stage, the inflation proceedsslowly and monotonously. Consequently, the outflow of fluid into thenext chamber in the series is also slow and monotonous. After reachingthe initial pressure peak, even for constant fluid inflow, the chamberinflates in size more rapidly, while at the same time, the inflatedpressure actually falls. As a result in this fall of internal pressure,the outflow of fluid into the next sequential chamber also falls, whileat the same time, the size of the first chamber is increasing morequickly. Ultimately, the expanding chamber inflates to such a size thatit meets the lumen wall, anchors there, and can no longer increase insize. From this point of time on, essentially all of the fluid inflowinto the chamber flows out into the next sequential chamber, such thatit then begins to inflate at the rate provided by the fluid source, andthe whole process is repeated for this next chamber.

This combination of events thus enables each chamber in the series toinflate and anchor at a faster rate than the inflation rate of theimmediately succeeding chamber. This feature enables the presentinvention to overcome a possible operative difficulty of prior art,single feed, sequentially inflatable chamber locomotion devices, in thatit is difficult to obtain the optimum conditions to combine rapidpropulsion speed with sequential inflation of the series of chambers. Insuch devices, if the orifices between chambers are too small, theinflation rate and propulsion speed are too low. If the orifices betweenchambers are too large, the chambers inflate so rapidly that there is atendency for the inflation to approach simultaneous filling conditions,and earlier chambers anchor onto the inside walls of the lumen beforethey have fully expanded axially, such that the increased inflation rateis not translated into an increased propulsion rate. The use of thenon-monotonous elastic material characteristic, according to the presentinvention, overcomes this possible problem by enabling the use ofcomparatively large connecting orifices between chambers so thatinflation occurs rapidly, but at the same time, introducing a delay inthe inflation of the subsequent chambers, such that a subsequent chamberdoes not inflate significantly until the previous chamber is properlyanchored in the lumen. This provides the desired combination of orderlyand rapid inflation and anchoring with a high propulsion speed.

The device according to the present invention is particularly useful inmedical applications for self-propulsion of a catheter through a lumen,by its tip. It can be applied in various medical fields such asEndoscopy, Gastro-entereology, Urology, Cardiology, Cochlearimplantation, sub-dural spinal applications, and others. Although theinvention is generally described in this application in terms of itsmedical application, it is to be understood that the invention is alsoequally applicable to non-medical applications, where vision,accessibility or maintenance are needed in passageways, such as inindustrial plant, gas pipes, power plants, tunnels, utility pipes, andthe like.

There is thus provided in accordance with a preferred embodiment of thepresent invention, a self-propelled device for locomotion through alumen, comprising:

-   (i) a set of serially arranged inflatable chambers, adjacent    chambers being connected by at least one connecting port providing    fluid communication between the adjacent chambers, and-   (ii) a fluid source attached to an end one of the set of serially    arranged inflatable chambers, such that the set of chambers inflate    sequentially,-   (iii) wherein at least one of the inflatable chambers has    characteristics providing a non-monotonous relationship between the    inflation pressure within the at least one chamber and its inflated    size.

In the above described device, the characteristics may comprise at leastone of the elastic properties of the elastic material, the wallthickness of the at least one chamber, and the uninflated size of the atleast one chamber. The characteristics may also be such that thenon-monotonous relationship has an inflation pressure peak at a firstinflated size. Additionally, the characteristics may preferably be suchthat the non-monotonous relationship has a negative slope when theinflated size of the at least one chamber is in a range immediatelylarger than the first inflated size. Within that range immediatelylarger than the first inflated size, the volume of the at least onechamber may preferably show an increase more rapidly than the volume offluid flowing into the at least one chamber, such that the inflationpressure of the at least one chamber falls. Furthermore, thecharacteristics may be such that the slope of the non-monotonousrelationship changes to a positive slope at a second inflated sizewithin the range immediately larger than the first inflated size. Thecharacteristics may be such that at the second inflated size, theinflation pressure goes through a minimum as a function of inflatedsize.

In accordance with still another preferred embodiment of the presentinvention, in any of the above described devices, chambers in the sethaving equal inflation pressures may have different inflated sizes.

There is even further provided in accordance with another preferredembodiment of the present invention, a device as described hereinabove,and wherein the characteristics are such that the at least oneinflatable chamber becomes fully inflated before a second chamberadjacent to the at least one inflatable chamber, and distal to the fluidsource, has inflated appreciably. In such a case, the at least oneinflatable chamber may become fully inflated by virtue of its contactwith a wall of the lumen.

In accordance with a further preferred embodiment of the presentinvention, the device may preferably be adapted for locomotion through abodily passage of a subject.

There is further provided in accordance with yet another preferredembodiment of the present invention, a method of sequential filling aset of serially arranged inflatable chambers, comprising:

-   (i) providing a set of serially arranged inflatable chambers,    adjacent chambers being connected by at least one connecting port    providing fluid communication between the adjacent chambers, and-   (ii) attaching a fluid source to an end one of the set of serially    arranged inflatable chambers,-   (iii) wherein the characteristics of at least one of the inflatable    chambers are selected to provide a non-monotonous relationship    between the inflation pressure within the at least one chamber and    its inflated size.

In the above described method, the characteristics may comprise at leastone of the elastic properties of the elastic material, the wallthickness of the at least one chamber, and the uninflated size of the atleast one chamber. The characteristics may also be such that thenon-monotonous relationship has an inflation pressure peak at a firstinflated size. Additionally, the characteristics may preferably be suchthat the non-monotonous relationship has a negative slope when theinflated size of the at least one chamber is in a range immediatelylarger than the first inflated size. Within that range immediatelylarger than the first inflated size, the volume of the at least onechamber may preferably show an increase more rapidly than the volume offluid flowing into the at least one chamber, such that the inflationpressure of the at least one chamber falls. Furthermore, thecharacteristics may be such that the slope of the non-monotonousrelationship changes to a positive slope at a second inflated sizewithin the range immediately larger than the first inflated size. Thecharacteristics may be such that at the second inflated size, theinflation pressure goes through a minimum as a function of inflatedsize.

In accordance with still another preferred embodiment of the presentinvention, in any of the above described methods, chambers in the sethaving equal inflation pressures may have different inflated sizes.

There is even further provided in accordance with another preferredembodiment of the present invention, a method as described hereinabove,and wherein the characteristics are such that the at least oneinflatable chamber becomes fully inflated before a second chamberadjacent to the at least one inflatable chamber, and distal to the fluidsource, has inflated appreciably. In such a case, the at least oneinflatable chamber may become fully inflated by virtue of its contactwith a wall of the lumen.

In accordance with yet a further preferred embodiment of the presentinvention, the above described method may further comprise the step ofusing said set of serially arranged inflatable chambers for providingself-propelled motion through a lumen.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description, taken in conjunction with thedrawings in which:

FIG. 1 illustrates schematically a prior art tip-propelled catheterdevice, such as that described in co-pending PCT Application No.PCT/IL2006/000925;

FIGS. 2A to 2I illustrates schematically how the fluid inflates theballoon cells of a device of the type shown in FIG. 1 in a sequence thatcauses the device to move forward;

FIGS. 3 to 5 are graphs of the pressure inside an inflatable elasticballoon, plotted against the radial expansion of the balloon, forballoons having different elastic inflation characteristics;

FIGS. 6A and 6B illustrate schematically a locomotion device accordingto a preferred embodiment of the present invention, comprising a seriesof inflatable balloons constructed of elastic material having thecharacteristics shown in the graph of FIG. 3; and

FIGS. 7 and 8 illustrate examples of inflation characteristics,experimentally determined to obtain the desired properties anddimensions of a set of inflatable balloons for use in a device fornavigating the colon of a subject.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is now made to FIG. 1, which illustrates schematically atip-propelled catheter device 15 for traveling down a lumen 10, as isknown in the art. The device may preferably comprise a number ofballoons 11 connected to each other by separators 12 with one or moresmall openings, preferably in the form of orifices 13 formed therein,such that all the balloons comprise a single volume, inflatable througha single input. For ease of construction, the device can alternativelyand preferably comprise a single inflatable balloon divided intoseparate balloon segments by separators with orifices such that theentire segmented balloon can be inflated through a single input. Theballoon fabric is preferably held in place relative to the separators 12by means of rings 17 or glued or molded to the separators. Whicheverpreferred construction is used, the device is connected by a single tube16 to a fluid supply for inflating the balloons or the balloon segments.For the sake of simplicity, the operation of the device will beexplained using the term balloon for each separate segment, although itis to be understood that the invention can equally be implemented usinga single balloon segmented to form the separate segments. The inflationfluid used can be any one of a compatible gas or liquid. The fluidsupply can alternatively be taken from the passageway through which thedevice is moving, by means of an on-board pump, and ejected theretoafter use.

Reference is now made to FIGS. 2A to 2I which illustrates schematicallyhow the fluid inflates the balloon cells in a sequence that causes theproximal one to inflate first, increasing its diameter as well as itslength. Being inflated, it locks itself against the inside walls of thetube, but at the same time, its increase in length advances the othercells which are not fully inflated yet and hence are not locked on theinside walls of the tubes. The cells are inflated in a sequence untilthe distal cell locks against the inner tube walls, but at a positionfurther along the tube than that of the un-inflated balloon distal cellinitial position. This situation is reached in FIG. 2E. The timing andorder of the sequence is mandated by the fluid flow dynamics through theorifices, and the dynamics of the balloon inflation. Disconnecting thesupply and allowing the fluid pressure to drop at this point, or pumpingout the fluid, as shown in FIG. 2F, causes the proximal cell to deflatefirst reducing both its length and diameter. Since the distal cell andall of the intermediary cells, are at this point still fully inflated,they are still locked against the inner walls of the tube, thus pullingthe proximal cell inward as the balloon deflates and decreases itslength.

The sequential motion series is repeated inducing motion of the entiredevice as can be seen in FIGS. 2A to 2I. The locomotion sequence iscomposed of two phases: inflation and deflation, with the arrows at theentrance of the inflation tube indicating the direction of fluid flow. Asimplified description of the dynamics of the sequential inflation is asfollows:

The flow through an orifice is proportional to the square root ofpressure difference across the orifice, and the square of the diameterof the orifice, such that the orifice sizes can be selected to providespecific inflation dynamics.

Inflation phase: Initially, the pressure is equal in each balloon and isequal to the outside pressure, therefore the balloons are in deflatedcondition, as in FIG. 2A. When the pressure in the supply tube rises,the fluid begins to flow through the first orifice into the first(proximal) balloon, as in FIG. 2B. The pressure difference between thefirst and second balloons is now lower than the pressure differencebetween the supply tube and the first balloon, such that the flow ratein the second orifice is slower and the second balloon inflates moreslowly than the first one. By this means, the pressure propagates in agradual manner to the last (distal) balloon until the pressure in allthe balloons is equal, as shown in FIG. 2E.

Deflation phase: Now the pressure in the supply tube is reduced to theoutside pressure, or the fluid is pumped out of the inflation tube, andthere is then a pressure drop between the supply line and the firstballoon. The fluid begins to flow out of the first balloon, as in FIG.2F. Again, since the pressure difference between the supply tube and thefirst balloon is greater than between the rest of the balloons, thefirst balloon deflates first, then deflates the second, and so on untilthe last balloon is deflated, as in FIG. 2I.

In a variation of the actuation sequence, it is possible to initiate thecycling process even before the last cell is fully deflated. In such acase there will always be a base point anchored to the passageway andhence will prevent unwanted slippage in the case of external forces.Different orifices sizes, or different numbers of orifices, can be usedbetween different positioned balloons to improve the locomotion andspeed of the device, all according to the dynamics of the fluid flow into, out of, and between balloons. Furthermore, the viscosity of theinflation fluid can be chosen to improve the locomotion dynamics.

As previously stated, the time delay between sequential inflation of theballoons is dependent primarily on the fluid impedance of the orificeconnecting neighboring balloons. However, it has been found in such adevice, that in spite of the apparent simplicity of the temporal controlof the inflation time delay between successive balloons, it is difficultto control the fluid flow to obtain acceptable performance of thedevice. If the orifices are too small, the whole inflation proceduretakes too long and the propulsion speed of the device is slow. If theorifices are too large, the pressure drop between neighboring balloonsis small, and since the radii of successive balloons is, for an elasticmaterial having a monotonous relationship between the internal pressureand inflation size, proportional to their inflation pressure, at anypoint of time during the inflation and deflation process, there islittle difference in diameter between successive balloons. Therefore,although successive balloons do inflate sequentially, the inflationsequence becomes so fast that it approaches simultaneous inflation, andis difficult to regulate. The same is applicable to the deflationprocess. Such a rapid inflation sequence may be disadvantageous in someapplications, because friction with the lumen wall of the rapidlyexpanding balloons may impede the axial expansion of the successiveballoons, and thus reduce the propulsion speed of the device. Thisproblem may be of great importance in such common applications aspassage through the gastro-intestinal tract, where the tendency of theinternal wall of the tract to contract elastically means that it willgenerally be in contact with the balloons as they inflate both radiallyand axially. With such prior art devices, there is therefore a trade-offbetween the speed of inflation of the successive balloons, by whichmaximum inflation speed would have been expected to result in optimumpropulsion speed of the device along the lumen, and the actuallyachieved propulsion speed, these factors sometimes working against eachother. It would therefore be desirable that, without reducing propulsionspeed, successive non-anchoring balloons should delay their inflationtime until only a short time before anchoring, such that their frictionwith the lumen wall remains low until a moment before anchoring. Such anarrangement would provide optimum propulsion speed along the lumen,since the inflation sequence would then more resemble a step sequence ofinflation, rather than a close-to-continuous and almost simultaneousinflation.. The object would thus be to provide a method of slowing downsuccessive balloon inflation without appreciably slowing down the wholeinflation sequence by restricting the orifice flow rate.

The above described performance of a serial set of inflating balloonsarises because of the use of balloons made of an elastic material andhaving dimensions such that they have a monotonous dependence ofinflation size with internal pressure. According to a preferredembodiment of the present invention, there is proposed a method ofconstructing the balloons of a serial set of inflatable balloons, whichenables the generation of such a desired delay in the inflation ofsequential balloons, such that the problem of almost simultaneousinflation can be solved, but without substantially reducing thepropulsion speed of the device. The novel construction is based onselection of the elastic properties of the balloon material, the size ofthe balloons and the balloon wall thickness, such that the balloons havea non-monotonous inflation characteristic with pressure.

Thus, for example, the well-known phenomenon that the pressure requiredto inflate a latex rubber party balloon is higher in the early stages ofinflation, than when the balloon is already partly inflated, is due, atleast in part, to such a non-monotonous characteristic of the balloon.(Additionally, part of the increasing ease of inflation is due to thebreaking of some of the monomer chains in the polymer during the firstinflation.)

The behavior of inflated balloons has been described in the book “Rubberand Rubber Balloons: Paradigms of Thermodynamics” by I. Muller and P.Strehlow, published as a volume in Lect. Notes. Phys., Vol. 637, (2004)by Springer Verlag, Berlin. In chapters 2 and 3 thereof, pages 7 to 34,herein incorporated by reference in their entirety, there is shown,inter alia, the derivation of the relationship between the inflationpressure of a balloon and its size, which is given on page 28, for aspherical balloon, by the formula:

$\begin{matrix}{\lbrack p\rbrack = {2\; s_{+}\frac{d_{0}}{r_{0}}\left( {\frac{r_{0}}{r} - \left( \frac{r_{0}}{r} \right)^{7}} \right)\left( {1 - {\frac{s_{-}}{s_{+}}\left( \frac{r}{r_{0}} \right)^{2}}} \right)}} & (1)\end{matrix}$

Where:

-   p is the inflation pressure of the balloon;-   s₊ and s⁻ are elastic constants of the balloon material, which are    dependent on the mass density of the material, the molecular mass of    the polymer chain making up the elastic material of the balloon    (about 120 isoprene molecules in the case of rubber), and the    temperature, as fully derived on pages 32 to 34 of the Muller book;-   d_(o) is the wall thickness of the balloon;-   r_(o) is the uninflated balloon radius; and-   r is the radius of the inflated balloon at pressure p.

Reference is now made to FIGS. 3 to 5, which show the pressure, measuredin atmospheres, inside an inflatable spherical elastic balloon, plottedagainst the radial expansion of the balloon, expressed as the ratio ofits inflated to its uninflated radius, r/r₀. The graphs are derivedusing equation (1), for a specific type of rubber, and each graph isplotted for a different value of the elastic parameter s⁻. FIG. 3 showsthe characteristic curve for s⁻=−0.3 bar, FIG. 4 for s⁻=−0.5 bar, andFIG. 5 for s⁻=−0.8 bar. As is observed, the behavior of the balloonduring inflation is dependent on the balloon material parameters, aswell as on the wall thickness, the balloon radius, and even on the shapeof the balloon, since the calculations for equation (1) are applicableonly for spherical balloons. FIG. 3 shows marked non-monotonous elasticbehavior, FIG. 4 shows very mild non-monotonous behavior, while theconditions of FIG. 5 show monotonous behavior.

According to a preferred embodiment of the present invention, theballoon material is selected, and the balloon dimensions and shape arechosen such that the balloons have a non-monotonous pressure/radiuscharacteristic, such as that shown in FIG. 3. Looking at the practicalconsequences of the inflation curve of FIG. 3, an initial inflationpressure is required to inflate the balloon to a certain inflationpoint, defined in this application as the initial inflation peak,designated 30 in FIG. 3. From that point onwards, there is a furtherinflation region with a negative inflation pressure coefficient, suchthat the balloon continues to inflate even though the pressure isreduced. In order to clarify this feature, it is to be emphasized thatthis is not meant to imply that the balloon will continue to inflateeven if no inflation fluid is added, but rather that for a given inflowof inflating fluid, the volume of the balloon will increase more rapidlythan the volume of fluid inflow, such that the internal pressure drops.This negative coefficient region persists as balloon radius increases,until a point of inflation is reached where the inflation pressurerequired to increase the radius reaches a local minimum, 31, defined inthis application as the intermediate inflation pressure minimum. Furtherincrease in inflated radius is only then achieved with an increase ofpressure. An internal pressure as high as the original initial inflationpeak, is only achieved during the continued inflation of the balloonafter a further increased inflation radius is reached, this being shownat point 32 in FIG. 3.

As a result of this behavior, there is a region of the inflationenvelope of the balloon, defined as that where the balloon has aninternal pressure between the initial inflation peak 30 and theintermediate inflation pressure minimum 31, in which there is no uniquevalue of balloon radius related to a predetermined inflation pressure.Thus, in the preferred example shown in FIG. 3, at a predetermined fixedpressure of, for example, 1.033 bar, there are three possible ballooninflation radii which can co-exist, at approximate values of r/r₀=1.25,2.1 and 4.7, the radius actually achieved being dependent on the historyof the inflation procedure. Thus, for a series of connected balloonshaving the same internal pressure, different balloons can have widelydifferent radii. This phenomenon is used in this embodiment of thepresent invention to enhance the behavior of the device by allowingsuccessive balloons to have smaller radii than the previously inflatedballoons for a longer period than that obtained with monotonous elasticmaterials, until pressure equilibrium is obtained. This is in contrastto the case of balloons having monotonous inflation characteristics,such as is shown in FIG. 5, according to which balloons having the sameor similar internal pressures will all have similar radii, such thatlarge orifices associated with high speed inflation would also havesmall pressure drops between balloons, and therefore similar internalpressures and hence similar inflation ratios.

Reference is now made to FIGS. 6A and 6B, which are schematicillustrations of a series of connected balloons, similar in structure tothose shown in FIGS. 1 and FIGS. 2A-2I of this application, but whereinthe balloon material has elastic properties such that the inflationcurve as a function of pressure has a non-monotonous form, such as isshown in FIG. 3. The balloon sizes are taken directly from computersimulations of the device. It is seen that in FIG. 6B, where the firstballoon has been just fully inflated, the other balloons are all onlyabout 1.2 times their initial radius, thus illustrating how theinflation process of the present invention operates as required.

If the fluid pumping rate into the first balloon, less the fluid outflowrate through the orifice to the following balloons, is such that theinflation pressure in the first balloon only reaches a pressure wellbelow the initial inflation peak, the balloon will not inflatesufficiently to perform its function well. This function ischaracterized by the need for the device to have sufficient radial spacefor a useful sized payload, and yet still to be slim enough whenuninflated to enable easy insertion into the lumen to be negotiated.This means that in practical terms, the ratio of the fully inflatedradius, r, to the uninflated radius, r₀, should preferably be at least1.5, and more preferably, at least 2.

In terms of the exemplary material and dimensions shown in theembodiment of FIG. 3, the fluid inflow rate relative to the fluidoutflow rate through the orifice should be such as to ensure that thepressure in the first balloon reaches the initial inflation peak 30. Upto this point, the first balloon grows at a rate determined by the netinflow of fluid. However, as soon as the pressure in the first balloonreaches the initial inflation peak, 30, the balloon will continuegrowing rapidly from a small net inflow of fluid, even though thepressure now falls down below the initial inflation peak because of therapid growth. The outflow of fluid into the second balloon does not yetresult in the second balloon reaching initial inflation peak, and thesecond balloon therefore inflates significantly more slowly than thefirst. Furthermore, the fall in pressure in the first balloon beyond theinitial inflation peak reduces the inflow of fluid into the secondballoon. At some point within this range beyond the initial inflationpeak, the first balloon will anchor onto the walls of the lumen, willcease expanding, and then all of the additional inflowing fluid willpass through the orifice to inflate the second balloon. Here, theprocess will be repeated again, with the second balloon expanding at itsslow rate until its initial inflation peak point has been reached, andthen at a faster rate until it anchors in the lumen. As a consequence ofthis action, even with a comparatively large orifice size to ensure asufficiently fast inflation cycle time for the entire series, thenon-monotonous elastic properties of the balloon material cause thesecond balloon to refrain from filling significantly until the firstballoon is fully inflated and anchored in the lumen. Therefore, incontrast to the continuous inflation process obtained using monotonouslyelastic materials, which, when the orifice is large enough, acquires analmost simultaneous character, when using the non-monotonous elasticmaterial construction of the present invention, the inflation processacquires a pulse-like inflation sequence, each balloon inflating in atemporally distinct operation, which is desirable for ensuringunfettered progress of the device through the lumen. The same sequenceof events operates in reverse when deflation is taking place.

Reference is now made to FIGS. 7 and 8 which illustrate two examples ofinflation characteristics, experimentally determined in order to obtainthe desired properties and dimensions of a set of inflatable balloonsfor use in a device for navigating the colon of a subject. The graphswere plotted by repeatedly inflating and deflating the balloon, andplotting diameter as a function of inflation pressure. The firstinflation path, 70 in FIG. 7, and 80 in FIG. 8, is markedly differentfrom the subsequent curves, and is not taken into account in theexperimental assessment of balloon characteristic suitability. Theballoons are preferably not spherical but cylindrical, having a largerlength than their diameter, such that they have a longer axial expansionwhen inflated. The device then moves further during each cycle, than adevice using spherical balloons of similar characteristics. In theembodiment whose results are shown in the graph of FIG. 7, the length ofthe balloon is 2.5 times the diameter, the diameter being 14 mm and thelength 35 mm. The balloon material is a silicone, type 4720, ofthickness 0.337 mm. Although there is a spread of the experimentalresults, the non-monotonous nature of the inflation characteristic isclearly seen, making this balloon a good candidate for a device whichwill move efficiently and speedily. Furthermore, for use in the colon,since an initial inflation peak of less than 100 mBar is desired, toprevent the possibility of pressure injury to the colon, the sampleshown in FIG. 7 complies with this requirement. It is found that thedeflation curve is slightly different from that of the inflation, asshown by the lower groups of lines in each graph.

In the example of FIG. 8, the balloon is made of the same material andthe same thickness as that of FIG. 7, but is essentially spherical,having a diameter of 14 mm. As is seen from FIG. 8, the inflation isalmost monotonous, such that this balloon, although made of identicalmaterial of the same thickness as in FIG. 7, is not suitable for use ina speedy and efficient device.

The various parameters of the balloons are varied in order to achievethe best non-monotonous result, within the limitations of the maximumpressure that can be applied to the balloon, this being dependent on theapplication in hand. Because of the complexity of the analytical form ofthe balloon inflation characteristics, as shown by equation (1) above,the experimental method outlined here is generally the simplest methodfor targeting the balloon properties required for each application.

In implementing the invention for different applications, variousparameters have to be taken into account in order to devise the mostsuitable device for the application. Thus, for use in thegastro-intestinal tract, the following general guidelines appear to beuseful:

-   Max.Pressure: 30-100 mBar. Use of inflation pressures above this    range in a self-propelled device in the colon may cause injury,    since the colon contracts onto any body within itself, and thus all    of the pressure within the balloons is transferred directly to the    colon wall.-   Diameter: 8-20 mm-   Balloon length: 10-50 mm., to provide good axial stroke.

In other body passages, the pressure range will be similar but thediameter may change according to the passage diameter.

It is to be understood that these exemplary embodiments and theirexperimental results are not meant to limit the invention in any way,but are brought only as exemplary embodiments of how the invention my beimplemented in some common medical applications. The device may also beused for remote access in industrial pipelines which it is required totraverse. For such industrial applications, there will generally be nocritical limitations about the maximum balloon pressure allowable, andeven more efficient and speedy devices may be designed than for medicalapplication.

It is appreciated by persons skilled in the art that the presentinvention is not limited by what has been particularly shown anddescribed hereinabove. Rather the scope of the present inventionincludes both combinations and subcombinations of various featuresdescribed hereinabove as well as variations and modifications theretowhich would occur to a person of skill in the art upon reading the abovedescription and which are not in the prior art.

1. A self-propelled device for locomotion through a lumen, comprising: aset of serially arranged inflatable chambers, adjacent chambers beingconnected by at least one connecting port providing fluid communicationbetween said adjacent chambers; and a fluid source attached to an endone of said set of serially arranged inflatable chambers, such that saidset of chambers inflate sequentially, wherein at least one of saidinflatable chambers has characteristics providing a non-monotonousrelationship between the inflation pressure within said at least onechamber and its inflated size.
 2. A self-propelled device according toclaim 1 and wherein said characteristics comprise at least one of theelastic properties of said elastic material, the wall thickness of saidat least one chamber, and the uninflated size of said at least onechamber.
 3. A self-propelled device according to claim 1, and whereinsaid characteristics are such that said non-monotonous relationship hasan inflation pressure peak at a first inflated size.
 4. A self-propelleddevice according to claim 3, and wherein said characteristics are suchthat said non-monotonous relationship has a negative slope when theinflated size of said at least one chamber is in a range immediatelylarger than said first inflated size.
 5. A self-propelled deviceaccording to claim 3 and wherein said characteristics are such thatwithin said range immediately larger than said first inflated size, thevolume of said at least one chamber increases more rapidly than thevolume of fluid flowing into said at least one chamber, such that theinflation pressure of said at least one chamber falls.
 6. Aself-propelled device according to claim 3 and wherein saidcharacteristics are such that the slope of said non-monotonousrelationship changes to a positive slope at a second inflated sizewithin said range immediately larger than said first inflated size.
 7. Aself-propelled device according to claim 6 and wherein saidcharacteristics are such that at said second inflated size, saidinflation pressure goes through a minimum as a function of inflatedsize.
 8. A self-propelled device according to claim 1 and whereinchambers in said set having equal inflation pressures can have differentinflated sizes.
 9. A self-propelled device according to claim 1 andwherein said characteristics are such that said at least one inflatablechamber becomes fully inflated before a second chamber adjacent to saidat least one inflatable chamber, and distal to said fluid source, hasinflated appreciably.
 10. A self-propelled device according to claim 9and wherein said at least one inflatable chamber becomes fully inflatedby virtue of its contact with a wall of said lumen.
 11. (canceled)
 12. Amethod of sequential filling a set of serially arranged inflatablechambers, comprising: providing a set of serially arranged inflatablechambers, adjacent chambers being connected by at least one connectingport providing fluid communication between said adjacent chambers; andattaching a fluid source to an end one of said set of serially arrangedinflatable chambers, wherein the characteristics of at least one of saidinflatable chambers is selected to provide a non-monotonous relationshipbetween the inflation pressure within said at least one chamber and itsinflated size.
 13. A method according to claim 12 and wherein saidcharacteristics comprise at least one of the elastic properties of saidelastic material, the wall thickness of said at least one chamber, andthe uninflated size of said at least one chamber.
 14. A method accordingto claim 12, and wherein said characteristics are such that saidnon-monotonous relationship has an inflation pressure peak at a firstinflated size.
 15. A method according to claim 14, and wherein saidcharacteristics are such that said non-monotonous relationship has anegative slope when the inflated size of said at least one chamber is ina range immediately larger than said first inflated size.
 16. A methodaccording to claim 14 and wherein said characteristics are such thatwithin said range immediately larger than said first inflated size, thevolume of said at least one chamber increases more rapidly than thevolume of fluid flowing into said at least one chamber, such that theinflation pressure of said at least one chamber falls.
 17. A methodaccording to claim 14 and wherein said characteristics are such that theslope of said non-monotonous relationship changes to a positive slope ata second inflated size within said range immediately larger than saidfirst inflated size.
 18. A method according to claim 17 and wherein saidcharacteristics are such that at said second inflated size, saidinflation pressure goes through a minimum as a function of inflatedsize.
 19. A method according to claim 12 and wherein chambers in saidset having equal inflation pressures can have different inflated sizes.20. A method according to claim 12 and wherein said characteristics aresuch that said at least one inflatable chamber becomes fully inflatedbefore a second chamber adjacent to said at least one inflatablechamber, and distal to said fluid source, has inflated appreciably. 21.A method according to claim 20 and wherein said at least one inflatablechamber becomes fully inflated by virtue of its contact with a wall ofsaid lumen.
 22. A method according to claim 12 and wherein saidcharacteristics comprise at least one of the elastic properties of saidelastic material, the wall thickness of said at least one chamber, andthe uninflated size of said at least one chamber.
 23. A method accordingto claim 12 further comprising the step of using said set of seriallyarranged inflatable chambers for providing self-propelled motion througha lumen.